I recently bought an Pentax 6x7 W/MLU with the metered ttl pentaprism, two lenses plus the 2X converter in a lot deal. I bought a couple brand new batteries for it and the meter works and is responsive to light changes. However the cameras mirror comes up when I advance the film, it comes back down when I release the shutter. I think I found the issue, there is small metal tab on the inside of the camera that doesn't quite come back up all the way. I pushed it the final way and the mirror went up and back down when I clicked the shutter. All the shutter speeds sound spot on too. Has anyone experienced this before and or know of anyone who could fix it? The camera is going to need new light seals as well. At this point I'm ready to put it up on EvilBay! Thanks for your time and help, if you need any close up pics of anything let me know. I've attached a picture of it with the 55mm lens attached. Sincerely, Mike Cable.

PS: I haven't removed and put the meter back on without first removing the lens.

Problems with these early cameras are common, often through long periods of inactivity; inaccurate TTL meters, shutter speeds (keep the shutter speed on X or B when the camera is not in use), winding problems, mirror/shutter solenoid and film seals all featuring prominently in serial reports of niggles that make them a bit of a gamble, even if subject to moderate use by enthusiasts. I am not a fan of the older cameras for any sort of use that requires reliability. Remember that 'way back when' these cameras were go-to workhorses in the studio or outside on location, and saw quite heavy service. How many years ago was that? Around 48 years from release to the present.

I gather from your description the the mirror is a bit sticky. The oddity is what you reported: that the mirror goes up when you advance the film and comes down when you press the shutter — that is not the correct operation at all. Sometimes a bit of exercise back and forth of that tab you found can be a fix. At other times, it is an echo of more involved problems under the hood like the winding mechanism.

If you, or a service person think this camera is repairable and worth holding onto, go for it. But the cost of repair could well exceed the net worth of the camera. That's the decision only you can make.
Parts must be taken from a like-camera, which introduces a cycle of redundancy and unreliability, even though it may give reasonably good service, if repair is thorough. But the later 1989-vintage Pentax 67 cameras (e.g. the one below) had a number of improvements underneath — not to say they are absolutely more reliable, because age will catch up with all of them eventually, but not as many problems have been observed with these later cameras. They do like to be used, but not abused.

I think that 55mm lens is the one before the present-day SMC Pentax 67 55mm — the naming convention differs from Takumars, to the next type and the final SMC Pentax 67 lenses. It should be just as sharp and contrasty as the current 55mm.

PS: Yes, that's correct about the mirror/TTL prim removal and reassembly; you can freely remove the prism, but before you re-seat it, remove the lens. Then re-seat the prism and re-mount the lens. This correctly lines up the aperture coupling chain buried in that little slot at the top front side of the focusing screen escucheon.